High-speed optical fiber communication systems operate by encoding information (data) onto lightwaves that typically propagate along optical fiber paths. Most systems, especially those used for medium to long distance transmission employ single mode fiber. As implied by the name, single mode fibers propagate only one mode of light below cutoff. The single mode typically includes many communications channels. The communications channels are combined into the one transmitted mode, as by wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM).
While only one mode is transmitted, that mode actually comprises two perpendicular (orthogonal) polarizations. The polarization of these two components varies undesirably as the waves propagate along a fiber transmission path. The distortion of the optical signals caused by the varying polarization is called polarization mode dispersion (PMD). PMD can be corrected through a combination of measurements of the PMD and the control of active corrective optics.
Polarimeters measure the polarization of light. Polarimeters can generate signals representing a measured degree of polarization that can be useful for diagnostic purposes. The signals can also be advantageously used for polarization correction using feedback techniques to minimize PMD.
Polarimeters generally employ one or more photodetectors and related electro-optical components to derive basic polarization data. The raw photodetector signal measurements are typically transformed by mathematical techniques into standard polarization parameters. In the prior art, the photodetector outputs are generally averaged, as by some electronic time constant, and then multiplied as part of the signal processing and transformation process. The problem with averaging at detection is that instantaneous temporal information lost through averaging cannot be retrieved later.
What is needed for more accurate polarization measurements is a polarimeter that instantaneously measures polarimeter photodetector outputs without averaging, multiplies the unaveraged signals early in signal processing, and then averages and transforms the signals into polarimetry parameters.